I live in a bedsit and my sink is connected to the one in the next room so we both use the same downpipe.

I have a bottle trap and the sink from the next room is connected to the downpipe with a T junction just below the level of my bottle trap. Although the T junction has a curve to direct the water down the pipe rather than allow it to hit the opposing side, it still tends to bubble up through my bottle trap making lots of noise and sometimes blowing back into my sink.

Can someone tell me if this DIY job is legal? I thought there was a regulation regarding how far below an existing trap a junction from another sink had to be.

First why do you thonk its a DIY job and second what if it is? No there is no such regulation provided the fall is correct there is nothing to say vertical separation distance.

Your problem is that the waste water from next door is creating a vacuum in the downpipe as it exits. This is sucking the contents of your trap out causing the noise sound. Wha is blowing back into your sink is not waste water from your neighbour its the contents of your trap.

Thanks for the advice. I'll get on to my landlord or his manager about it.

The reason I said it was a DIY job is that neither my landlord nor the manager who does all the work around here is a qualified plumber. I know from talking to the manager that they converted the house into bedsits between them. Had they been qualified, they would probably have know about these traps or at least, the problems associated with having a 2 into 1 sink drainage system.